GSM Data Services
From Circuit-Switched voice to Packet-Switched broadband. An exploration of how 2G GSM networks evolved to carry the internet.
Circuit Switched Data (CSD)
The original GSM data service. A dedicated circuit is established for the duration of the call.
- Speed: 9.6 kbps
- Billed per minute
- Occupies voice channel
General Packet Radio (GPRS)
The "2.5G" revolution. Data is split into packets and transmitted only when needed.
- Speed: Up to 171.2 kbps
- Billed per volume (MB)
- "Always On" connection
Why the shift to Packet Switching?
In traditional voice calls (Circuit Switched), the network reserves a fixed bandwidth path for the entire duration of the call, even during silence. This is inefficient for bursty data (like web browsing).
Packet Switching (GPRS) utilizes the idle time slots in the GSM frame. It only uses the radio channel when there is data to send, allowing the operator to serve data users dynamically without blocking voice calls.
171 kbps
GPRS Max Speed
9.6 kbps
CSD Speed
~18x Faster
Evolution: From CSD to HSCSD
Before the packet-switching revolution of GPRS, the GSM standard evolved to provide faster data rates through circuit-switched enhancements. High Speed Circuit Switched Data (HSCSD) was the critical bridge technology.
CSD (Circuit Switched Data)
Original GSM (1991)
Speed
9.6 kbps
Technology
Single Timeslot
Billing
Per Minute
Uses one traffic channel (TCH) per user. Error protection uses 1/2 rate convolutional coding, limiting the data rate to 9.6 kbps.
HSCSD (High Speed CSD)
2G+ Enhancement (1999)
Max Speed
57.6 kbps
Technology
Multi-Slot + TNT
Billing
Per Minute (Higher cost)
Key Innovation: Multi-Slot
Instead of using 1 timeslot, HSCSD allows a user to bind multiple timeslots (up to 4 in downlink, 4 in uplink) to a single connection.
4 slots × 14.4 kbps = 57.6 kbps
Key Innovation: TNT
Tandem Free Operation / Transparent Network Transmission reduces error protection (from 1/2 rate to less) when the radio link is good, increasing the raw bit rate per slot to 14.4 kbps.
Why did HSCSD fail to catch on?
Despite being faster than CSD, HSCSD was still Circuit Switched. It was expensive (billed by time), inefficient for bursty internet traffic, and required the network to dedicate multiple voice channels to one user, reducing capacity for voice calls. GPRS solved these issues by moving to packet switching.
Core Theory: GPRS Architecture
GPRS requires new network elements to be added to the standard GSM architecture to handle packet data. It reuses the Base Transceiver Station (BTS) and Base Station Controller (BSC) but adds core network nodes.
Mobile Station
BTS
Base Transceiver Station
BSC
Base Station Controller
PCU (Packet Control Unit)
SGSN
Serving GPRS Support Node
- Mobility management
- Session management
- Connection to BSC
GGSN
Gateway GPRS Support Node
- Gateway to Internet (PDN)
- IP address assignment
- Firewall & Routing
Internet (PDN)
Coding Schemes (CS)
GPRS uses four different coding schemes to protect data against errors. Better protection means lower throughput.
| Scheme |
Protection |
Data Rate |
| CS-1 |
High (1/2 rate) |
9.05 kbps/slot |
| CS-2 |
Medium |
13.4 kbps/slot |
| CS-3 |
Medium |
15.6 kbps/slot |
| CS-4 |
None |
21.4 kbps/slot |
Multislot Class
Defines the number of timeslots (channels) a device can use simultaneously.
Class 2
1 Downlink, 1 Uplink
Class 10 (Common)
4 Downlink, 1 Uplink
Class 12
4 Downlink, 4 Uplink
* Max theoretical speed = Slots × Rate (CS-4). 4 slots × 21.4kbps ≈ 85kbps (real world ~40kbps).
EDGE
Enhanced GPRS
EDGE (also known as EGPRS) is a bolt-on enhancement to 2G and GPRS networks that allows for higher data transfer rates. It is often referred to as "2.75G".
8-PSK Modulation
While GPRS uses GMSK (1 bit per symbol), EDGE uses 8-PSK, encoding 3 bits per symbol. This triples the gross data rate.
Link Adaptation
EDGE selects from 9 Modulation and Coding Schemes (MCS-1 to MCS-9) dynamically based on radio quality, maximizing throughput.
Incremental Redundancy
Instead of resending a whole block if it fails, EDGE sends additional parity bits to help decode the original message.
Modulation Comparison
GMSK (GPRS)
Gaussian Minimum Shift Keying. Phase shifts by 90 degrees. Robust but limited to 1 bit per symbol.
8-PSK (EDGE)
Phase Shift Keying with 8 phases. Allows 8 distinct states (2^3), encoding 3 bits per symbol.
Evolution of Speed
Comparing the theoretical maximum download speeds of GSM data technologies, including HSCSD.